The Burning Edge points out that you can now search in textboxes and textareas in a webpage when you do a "Find in this page" search. The corresponding bug is bug 58305 and this enhancement appears in builds on and since 2004-04-16. I sorely missed this, especially so because IE does have this feature - some sites use textareas to display code.
As pointed out in this comment, this works for both the "Find in this page" function (accessible via Ctrl-F or Edit -> Find in this page...) and the "Find as you type feature". But with "Find as you type", the text found is not highlighted due to bug 134586 - selection in text field is invisible when it doesn't have focus. This will probably be fixed by the time Firefox 1.0 is released.